Apparatus and method for locating events are disclosed in our U.S. Pat. Nos. 6,621,947 and 6,778,717, and U.S. patent application Ser. No. 11/311,009. The contents of these two patents and the application are incorporated into this specification by this reference.
The system used in the above-identified U.S. patents utilises a bi-directional Mach Zehnder (MZ) interferometer in which counter-propagating signals are provided. By measuring the time difference of perturbed signals caused by an event, the location of the event along the sensing device formed by the Mach Zehnder interferometer can be determined.
Thus, when an event perturbs the MZ sensor portion of the system, the difference in the arrival time of the counter-propagating signals at the detectors can be used to calculate the exact location of the perturbation on the MZ sensor. This type of sensor can be applied to perimeter or infrastructure security applications, with typical sensing lengths exceeding 50 km.
The above U.S. application specifically discloses improvements to the concept disclosed in the U.S. patents by adjusting the input polarisation states to phase match the counter-propagating optical output signals. By matching the phase of the counter-propagating optical output signals, output fringes at the detectors are produced which are easily detected and therefore, the time difference between receipt of the two modified counter-propagating signals can be accurately recorded to more accurately determine the location of an event.